Spectrometers function using the principle of dispersion of light, which occurs when rays of light are deviated, typically by a diffraction grating, or refracted through a prism. Diffraction gratings behave optically like a multiplicity of very narrow individual slits, which cause light rays to be deviated at angles depending upon the wavelength of those rays.
In existing spectrometers, light transmitted through a slit is dispersed using a diffraction grating or prism. The dispersed light is then imaged onto a detection focal plane, which typically contains an array of minute photosensitive elements. Most spectrometers include a collimator to make all the light rays incident on the grating or prism parallel. Collimating is necessary to make sure all the rays of the same color are dispersed at the same angle. However, current spectrometers are relatively bulky instruments since the optical ray paths tend to be fairly long. In addition, current scanning spectrometers require mechanical motion and rotate the dispersing element to scan wavelengths past the detectors. These mechanical motions are sensitive to vibrations and result in wear, which may cause alignment and/or calibration problems.
Many prior art devices used for wavelength measurement require a stable, precise light source for generating a stable beam of light having a narrow spectrum centered about a particular wavelength. For example, U.S. Pat. No. 4,989,039, incorporated herein by reference, provides and uses a mercury lamp having a wavelength of 253.7 nm as a reference light. This and other systems, which rely on, a precise reference wavelength with which some comparison is performed to determine a wavelength of an input signal significantly adds to the cost of the device. Furthermore stabilizing such light sources is difficult and costly.
Fixed fiber Fabry-Perot (fixed FFP) filters can be used as accurate wavelength references for the calibration of optical spectrum analyzers (OSA) to increase both accuracy and resolution of measurements. Although fixed FFP filters produce multiple, very accurately spaced, wavelengths (i.e. a comb of peaks), a consistent problem has been the difficulty of accurately identifying an individual wavelength among the multiple wavelengths produced.
Fiber Fabry-Perot tunable filters (FFP-TF) have been successful commercialization for use in he first wavelength detection multiplexing (WDM) systems and have demonstrated robust and field-worthy operation. WDM systems have rapidly developed moving to 8, 16 and 32 (and higher) wavelength systems using other less expensive demultiplex technology. These developments have made interrogator systems for accurately measuring the wavelength response of passive fiber optics devices possible. Tunable FFP filters can be used as the needed OSA component in such dense WDM, if suitable methods for wavelength referencing and calibration can be found. For dense WDM systems, the accuracy of absolute wavelength measurements is preferably about 0.5 to about 0.1 nm or higher, power measurements are preferably about 0.1 dB and signal-to-noise measurements are preferably about 1 dB or less. Fixed FFPs and FFP-TFs are described, for example, in U.S. Pat. Nos. 5,212,745; 5,212,746; 5,289,552; 5,375,181; 5,422,970; 5,509,093 and 5,563,973, all of which are incorporated by reference in their entireties herein, particularly for their disclosure of the structures and operation of these filters.
In-fiber Bragg gratings (FBGs) have been used in fiber optic sensors for strain and temperature measurements. These sensing techniques depend on the ability to accurately measure the wavelengths of light reflected or transmitted by FBGs in a sensor fiber.
Fiber Bragg gratings can produce a narrow-band response around a single wavelength (reflecting a narrow-band peak or passing the illuminating spectrum with a narrow-band notch or hole). Dopants are used to increase the index of refraction in the cores of optical fibers are photosensitive. By exposing a single-mode fiber to interfering beams of UV light or through a suitable mask, a diffraction pattern can be written into the core that reflects a single narrow-band wavelength of light. The resulting fiber Bragg grating (FBG) passes all other wavelengths carried by the single-mode fiber and reflects almost all (up to 99.9%) of the light that meets the Bragg condition (.lambda.=2s, the Bragg reflection wavelength), where s is the spacing of the grating. If the FBG is mounted on a structure (typically a structure much larger than the grating itself), then the spacing of the grating and the corresponding reflected wavelength of the FBG are affected by and can be used to sense strain, temperature, pressure, etc. in the structure depending on the mounting configuration. A sensor system is constructed by creating a number of FBGs (typically each of different Bragg wavelength) spaced along a single optical fiber to generate a highly multiplexed sensor system reflecting at different wavelengths. Sensor systems can also be constructed in which multiple FBGs (of different wavelength) are created in the same location on the fiber.
FBGs have recently become widely commercially available at relatively low cost and are projected to be extensively used in multi-wavelength telecommunications systems. Thus, there is a growing need for devices and methods for interrogation of sensor, telecommunication and related systems that employ FBGs. In particular, there is a need for devices and methods that provide precise, accurate and reproducible determination of wavelengths reflected (or alternatively of the notches transmitted) by FBGs.
U.S. Pat. No. 5,838,437 in the names of Miller et al, entitled Reference System for Optical Devices Including Optical Scanners and Spectrum Analyzers, incorporated herein by reference, hereafter called the '437 patent, discloses an optical spectrum analyzer which utilizes a fixed Fabry-Perot etalon and a Bragg grating, as does the instant invention. Notwithstanding, one drawback to the '437 patent is that it does not provide a real-time or near real-time analysis of an optical signal to be analyzed. For example, it is a goal of the instant invention, to provide an optical signal to the device and to obtain a real-time or nearly real-time indication of its wavelength. By real-time, what is meant is not real-time processing but simultaneous receipt upon the diffraction grating or dispersive element of a reference signal and the test signal. The '437 reference in contrast to the instant invention ensures that a particular condition must be met in the process of analyzing an optical signal; In the '437 reference switching is invoked between the reference signal and the signal under test. When one signal is utilized, the other is not, and vice versa. Hence the system switches or alternates between the reference signal and the test signal but does not simultaneously utilize these signals. Advantageously, in the instant invention it is essential for the signal under test and the reference signal to be simultaneously impinging on the dispersive grating. By so doing, preferably the same conditions, i.e. substantially same optical path and grating angle are experienced by both signals, even in the presence of unwanted vibrations or other severe conditions.
In contrast to the '437 reference, the instant invention simultaneously provides a reference signal and the test signal and utilizes the reference signal in a real-time manner to determine the wavelength of the test signal. Hence, there is no delay between the receipt of the reference and test signals by their respective detectors, and the two signals simultaneously impinge upon a same grating or dispersive element; it is preferable but not essential that the two signals impinge at substantially the same angles of incidence.
In order to process the test signal and the reference signal through the same spectral dispersion element, the two signals are both disposed adjacent one another, more specifically, stacked on top of one another, and directed to a focusing means in the form of a single or plural lenses or concave mirrors such that they use the same dispersion element simultaneously.
Advantageously, the present invention is relatively inexpensive to manufacture and allows an inexpensive flexure element to be used as a means of moving the diffraction grating. The flexure element need not be linear in its movement however must be monotonic within a working range.
Advantageously, since the wavelength tracking optics are reliable, a simple, inexpensive movement can be used while obtaining accuracy of wavelength measurement.